Arrangements for controlling the lateral position of a web of material

ABSTRACT

An apparatus for controlling the lateral position of a web of material, having a sensing head with three pneumatically operated sensing elements each of which senses the presence or absence of the web in the head. First and second sensing elements are nominally disposed with one longitudinal edge of the web therebetween and the third sensing element is disposed inwardly of the element nominally sensing the presence of the web so as to do likewise. Signals from the first and second elements are fed to a control means which activates an adjusting device to maintain the web in a desired lateral position while signal from the third sensing element indicating absence of the web is used to inhibit the operation of the adjusting device.

United States Patent [54] ARRANGEMENTS FOR CONTROLLING THE LATERAL POSITION OF A WEB OF MATERIAL 14 Claims, 2 Drawing Figs.

[52] U.S. Cl. 226/18, 226/45, 226/22 [51] Int. Cl B65h 25/08 [50] Field of Search 226/45, 22

18; 73/37.7, 37.8; 137/33, 84; ZOO/61.13, 83.32

Primary Examiner-Allen N. Knowles Assistant ExaminerGene A. Church Attorney-Rich and Ericson ABSTRACT: An apparatus for controlling the lateral position of a web of material, having a sensing head with three pneumatically operated sensing elements each of which senses the presence or absence of the web in the head. First and second sensing elements are nominally disposed with one longitudinal edge of the web therebetween and the third sensing element is disposed inwardly of the element nominally sensing the presence of the web so as to do likewise. Signals from the first and second elements are fed to a control means which activates an adjusting device to maintain the web in a desired lateral position while signal from the third sensing element indicating absence of the web is used to inhibit the operation of the adjusting device.

P ATENTEDMAR 2mm SHEET 1 or 2 ATTORNEY.

PATENTED MAR 219m sum 2 0F 2 \wmhw Pm Mm mm H 4 m y w INNENTOR @MW MMWJ ATTORNEY ARRANGEMENTS FOR CONTROLLING THE LATERAL POSITION OF A WEB OF MATERIAL BACKGROUND OF THE INVENTION In many situations where a web of paper, cloth or other material is passed through a machine it is desired to control the lateral position of the web in the machine within close limits.

One way of performing this control has been to arrange two pneumatic sensing elements adjacent the desired lateral position of one edge of the web. The desired position of the web is such that its edge is between the two sensing elements so that one sensing element indicates the presence of the web while the other indicates the absence of the web.

If the web moves laterally such that both sensing elements indicate absence of the web, then this may be taken as an indication that the web has moved away from both sensing elements. The sensing elements are so coupled to a control system that the absence of the web at both sensing elements causes an adjustment device to move the web laterally until one of the sensing elements again indicates the presence of the web. i t i On the other hand, if both sensing elements become covered by the .web and hence indicate its presence, the web has inadvertently moved too far towards the sensing elements. In this situation the adjusting device is made to operate so that the web is driven back towards its desired position whereupon the other of the sensing elements will indicate the absence of the web.

A disadvantage of this arrangement is that substantial local discontinuities in the edge of the web tend to be followed by the sensing elements. Generally, in the presence of local edge discontinuities, it is better that the web should follow a straight I line rather than that it should be guided by these irregularities.

This is particularly so in the case of discontinuities such as slits extending part of the distance across the web.

A general object of the invention is to overcome this problem and provide an improved apparatus for controlling the lateral position of webs of material.

SUMMARY OF THE INVENTION elements, said sensing elements being so positioned as to define the limits of a range of permitted lateral deviation in the position of one longitudinal edge of the web control means responsive to signals from said first and second sensing elements, and an adjusting device controlled by said control means to cause movement of the web to maintain the latter in its desired lateral position, the improvement comprising a third pneumatically operated sensing element positioned to sense the presence of the web at both of said limits of lateral deviation and operably connected to said pressure medium supply means and to said control means which inhibits operation of the adjusting device when receiving a signal from the third sensing element indicating absence of the web.

The invention comprises the improvement of a third pneumatically operated sensing element positioned to sense the presence of the web at both of said limits of lateral deviation and connected to said pressure medium supply meansand to said control means which inhibits operation of the adjusting device when receiving a signal from the third sensing element indicating absence of the web. a a

A substantial discontinuity in the edge of the web results in the absence of web at the location of the third sensing element. In the absence of the third sensing element and associated control means, the fact that both the first and second sensing elements are uncovered by the web would have hitherto resulted in undesired adjustment of the lateral position of the web. The presence of this third sensing element now prevents any adjustment from taking place until the discontinuity has been passed. In this way the web tends to run straight, even in the presence of substantial discontinuities along the edge of the web.

Preferably the sensing head is in the form of a block having a slot therein through which the web passes, the pressure medium supply means is a source of compressed air and each sensing element is in the form of an air emitting nozzle communicating with said'slot, an aperture disposed in axial alignment with said nozzle and a tapping extending into said aperture to sense changes in pressure therein.

Preferably the control means includes a plurality of turbulence amplifiers which are controlled by the changes of pressure sensed by the tappings.

Various other aspects and features of the invention will become more apparent form consideration of a constructional embodiment thereof.

BRIEF DESCRIPTION OF DRAWINGS A constructional embodiment of the invention will now be described by way of example only, with reference to the accompanying drawings in which:

FIG. 1 is a plan view of a web, indicating the relative positions of three sensing elements with respect to the desired position of the web;

FIG. 2 is a pneumatic circuit diagram of an apparatus for controlling the position of the web.

DESCRIPTION OF PREFERRED EMBODIMENT FIG. 1 shows part of a web of material. 26 having an edge 12 incorporating a substantial discontinuity 13 in the form of a slit. The actual material of the web 26 is not important from the point of view of this invention and suffice it to say that the invention can be utilized with any substantially continuous lamellar body of suitable dimensions. In practice, the web 26 is drawn past a sensing head incorporating three sensing elements 14, 1S and 16 but for convenience, this situation is represented diagrammatically in FIG. 11; by a series of lines designated 14, 15 and 16 which represent the positions of the sensing elements relative to the web 26 when the latter has moved over a period of time.

Over the range of time represented by the upper part of ,FIG. 1 when the web 26 is in its desired lateral position and moving longitudinally the first element 14 senses the absence of the web 26, the second element 15 senses the presence of the web 26, and the third element 16 senses the presence of the web 26. Any slight deviation of the edge of the web 26 between the positions of the elements 14 and 15 has no effect on an associated adjusting mechanism, hereinafter described, and thus the web 26 continues to move in a longitudinal direction without adjustment. In contrast, if the web 26 should move laterally sufficiently far away from the element 14 to pass beyond the zone of the element 15, this element 15 now senses the absence of the web 26 and this in turn will result in a corresponding adjustment of the lateral position of the web 26 until the web 26 again covers the element 15. Similarly, if the web 26 deviates in the other reliability direction to such an extent as to cover the element 14 which then senses the presence of the web 26, the web will be moved laterally until the element 14 again senses the absence of the web 26. In this way, the control system maintains the pertinent edge of the web 26 between the two elements 14 and 15.

In the time period represented by the lower part of FIG. 1, the substantial discontinuity represented] by the slit 13 passes through the sensing head. In the absence of the third sensing element 16, the signals from sensing elements 14 and 15 would erroneously indicate that the web 26 had moved away from the second sensing element 15 and thus a correction would be applied to move the web 26 towards the left of the figure. In this case when the end of the slit 13 is reached, with the web 26 displaced laterally to the left of the figure by a substantial amount, the sensing elements 14' and 15 would correctly sense this deviation from the desired position, and would effect the appropriate correction to move the web 26 back to the right of the figure. I

This substantial movement of the web 26 first in one direction and then in the opposite direction is obviously not desirable.

In order to obviate this undesirable movement a third sensing element 16 is incorporated in the sensing head and this sensing element 16 is positioned so that normally with a web 26 having a straight edge the element 16 always senses part of the web 26. However, in the presence of a substantial discontinuity such as the slit 13, the element 16 sense the absence of the web 26 and this is indicative of a discontinuity rather than a deviation of the position of the web 26 as a whole. The associated control means is such that when the absence of web 26 is signalled by the element 16, the lateral adjustment device does not act to move the web 26 and no lateral adjustment can takeplace until the discontinuity has passed the element 16.

The third sensing elements 16 should be so positioned relative to the sensing elements 14 and 15 that it will reliably indicate the presence of a substantial discontinuity in the web 1 1.

A complete apparatus by means of which the sensing and associated control junction may be achieved is indicated diagrammatically in FIG. 2. The apparatus has a sensing head 50 which is constituted by a metal block having an air inlet passage 21 communicating with three nozzles 22, 23 and 24 which form part of the sensing elements 14, 15 and 16 of FIG. 1 respectively. The nozzles 22, 23 and 24 open into a slot 25 in the metal block within which a web 26 is moved. On the opposite face of the slot 25 from the nozzles 22, 23 and 24 are three apertures 27, 28 and 29 extending through the metal block to communicate with the atmosphere. Tappings or passages 31, 32 and 33 extend into the apertures 27, 28 29 and are used to sense changes of pressure in the apertures 27, 28 and 29 and thus to indicate the presence or absence of air flow from the nozzles 22, 23 and 24 to the apertures 27, 28 and 29. A .restrictor 47 is provided in the air supply passage 21 to the head 50 in order to restrict the prevailing pressure in the head 50.

The tappings 31, 32 and 33 are connected by way of tubes 34, 35 and 36 respectively to a series of turbulence amplifiers H1, H2, H3, H4 and H forming part of the control means.

A filter 37 is fed from pressurized air and serves to pass air under pressure to the remainder of the apparatus. In particular, the filter37 feeds air to the restrictor 47 and to a pressure regulator 38 which controls the pressure at which air is supplied into a manifold 39. The manifold 39 provides a common supply all five turbulence amplifiers H1 to H5 and the pressure in the manifold 39 is indicated by a herein in 46.

The outputs from turbulence amplifiers H1 and H5 are used directly to control step-up relays 41 and 42 respectively and these step-up relays in turn control the supply of air to a double acting piston and cylinder unit 43 which is connected to any conventional device 60 for adjusting the lateral position of a web. Devices for adjusting t the lateral position of the web are well known in themselves and for this reason are not described herein detail.

The filter 37 also feeds air to a pressure regulator 44 and the pressure at the side of the regulator 44 remote from the filter 37 is monitoredby a gauge 45. The actual supply of air to the unit 43 is by way of sprung-return three-way valves 51, 52. The valves 51, 52 have pilot or control inputs which are fed from the step-up relays 41, 42 respectively. The valves 51, 52 are connected to the regulated air supply provided by the regulator 44. The valves 51, 52 are shown in the position where there is no control signal from either of the relays 41, 42 and there is no air in the system.

The valves 51, 52 are connected to the unit 43 and to an exhaust outlet 53, 54 respectively The exhaust outlets 53, 54 are connected to restrictors 55, 56 respectively which lead to the atmosphere. When one side of the piston of the unit 43 is pressurized the other side of the piston will exhaust air which can vent to the atmosphere via the valves 51, 52 and restrictors 55, 56.

The restrictors 55, 56 control the speed of movement of the piston of the unit 43. The provision of the valves 51, 52 ensure that exhaust air from the unit 43 is not passed back into the step-up relays 41, 42 and thereby. contamination of these relays 41, 42 is prevented.

The manually operable jogging valves 48 and 49 are provided to control the turbulence amplifiers H1 and H5 in order to permit manually operable jogging controlled lateral displacement of the web 26. The jogging valves 48, 49 preferably have a pushbutton control.

Of the turbulence amplifiers, H1 is employed directly to control the opening and closing of the step-up relay 41. The turbulence amplifier H1 is in turn controlled by two further turbulence amplifiers H2 and H3. Similarly, the turbulence amplifier H5 is employed directly to control the step-up relay 42 and this amplifier H5 is in turn controlled by the turbulence amplifier H4. There are also direct connections between the jogging valves 48 and 49 and the amplifiers H1 and H5 respectively.

The five turbulence amplifiers H1 to H5 are each constructed to operate so that in the absence of any control signal, a substantial pressure appears at the output of the amplifier. However, if any control signal is applied to the amplifier this disturbs the flow from the input to the output of the amplifier and a much reduced pressure then appears at its output.

There are four distinct possible combinations of signals from the sensing head 50 and the effect of these and the jogging valves together with the subsequent operation of the control means and adjusting device will now be considered for each individual case.

1. FIRST SENSING ELEMENT 14 SENSES ABSENCE,

SECOND AND THIRD SENSING ELEMENTS 15 and 16, SENSE PRESENCE.

In this condition, there is a pressure signal in line 34 but not in lines 35 and 36. The signal in line 34 inhibits an output signal from amplifier H4 so that there is no inhibiting control signal to amplifier H5. The output signal pressure from the amplifier H5 operates the step-up relay 42 to pressurize the right-hand side of the piston of the unit 43 via the valve 52.

At the same time, there is no inhibiting signal to amplifier H3 and thus the output signal pressure from the amplifier H3 inhibits operation of the amplifier H2. With no signal pressure at the output of the amplifier H2 there is no inhibiting control signal for the amplifier H1 and thus the output signal pressure from the amplifier H1 operates step-up relay 41 to pressurize the left-hand side of the piston of the unit 43, via the valve 51.

Thus, with both sides of the piston of the unit 43 pressurized a balance is achieved and no control operation takes place.

2. FIRST, SECOND AND THIRD SENSING ELEMENTS 14, 15, 16, ALL SENSE PRESENCE.

In this case the web 26 is too far to the left of FIG. 1 and an appropriate adjustment is required to move the web 26 to the right of FIG. 1.

Under these conditions there is no signal pressure in any of the lines 34, 35 and 36. There is hence no inhibiting signal applied to the amplifier H4 and thus the output signal pressure from this amplifier is applied to the control input of amplifier H5 and this in turn inhibits an output signal from the amplifier H5.

There is no inhibiting signal from amplifier H3 and thus the output signal pressure from this amplifier inhibits amplifier H2. In the absence of a signal from the amplifier H2, there is no inhibiting signal to the amplifier H1 and thus the output signal pressure from the amplifier H1 operates the step-up relay 41.

Thus, there is a state of imbalance with pressure applied through step-up relay 41 and the valve 51 to the left-hand side of the piston of the unit 43. This pressure imbalance causes the piston of the unit 43 to be displaced which in turn causes the adjusting device to move the web 26 towards the right to restore the situation in which the first sensing element 14 senses the absence of the web 26. When this occurs the same conditions apply as in case land accordingly theunit 43 is in a state of balance.

3. FIRST AND SECOND SENSING ELEMENT 14 and 15 SENSE ABSENCE, THIRD SENSING ELEMENT 16 senses PRESENCE. y

This is the situation that occurs when the web 26 is slightly too far to the right of FIG. 2 and correction is required to move the web 26 towards the left. f

In this condition both air lines 34 and 35 are pressurized while air line 36 remains unpressurized. Line 34 applies a signal to amplifier H4 to inhibit the output signal pressure from this amplifier. The result is that amplifier H5 is not inhibited and a signal pressure is present to operate the step-up relay 42.

The signal on line 35 inhibits operation of amplifier H3 and thus there is no control signal to inhibit operation of amplifier H2. The signal pressure from amplifier H2 inhibits operation of amplifier H1 which in turn results in no signal being applied to step-up relay 41.

Thus there is a state of imbalance with pressure applied through the step-up relay 42 and the valve 52 to the right-hand side of the piston of the unit 43 which results in movement of the web 26 in a leftward direction until the sensing element 15 again senses the presence of the web 26.

4. FIRST, SECOND AND THIRDSENSING ELEMENTS 14,15 and 16, ALL SENSE ABSENCE.

This is the situation that exists when a substantial discontinuity such as the slit 13 of FIG. 1 is encountered so that the third sensing element 16 as well as the first and second sensing elements 14 and 15 sense absence of the web.

In this situation, there is a signal present on line 36 as well as lines 34 and 35. The signal in line 36 inhibits operation of both amplifiers H2 and H4. Thus there is no signal applied to either amplifier H1 or H5 and correspondingly both step-up relays 41 and 42 are operated. This results in a pressure balance between the two sides of the piston of the unit 43 and thus no control operation takes place.

5. MOVEMENT OF THE WEB UNDER MANUAL CON- TROL OF PUSHBUTTON JOGGING VALVE 48 or 49.

If jogging valve 48 isoperated an inhibiting signal is supplied direct to the control input of amplifier HS and a similar inhibiting signal is applied to the amplifier HZQWith no output signal from amplifier H2, there is no inhibiting signal for the amplifier H1 and thus the amplifier H1 supplies an output signal pressure to step-up relay 41 while the step-up relay 42 remains unoperated. The left-hand side of the piston of the unit 43 is pressurized and this causes the adjusting device to move the web 26 to the right of FIG. 1.

Similarly, manual operation of the jogging valve 49 supplies an inhibiting signal to the amplifier H1 and to the amplifier H4. In the absence of output signal pressure from the amplifier H4 there is no inhibiting input of the amplifier H5. Thus, in these circumstances the stepsup relay 42 is operated while the step-up relay 4] remains unoperated. The right-hand side of the piston of the unit 43 is thus pressurized resulting in movement of the web 26 to the left of FIG. 1.

In order to adjust the sensitivity of the control system it may be desired to adjust the lateral spacing of the first and second sensing element 14 and 15. A simple way of doing this is to mount the sensing head 50 as a whole so that it is pivotal about a vertical axis. Pivotal movement of the head 50 about this axis then results in adjustment of the lateral distance between the first and second sensing elements 14 and 15 and also, but this is less important, of the lateral spacing between the sensing elements 15 and 16.

A problem which frequently arises in the use of pneumatic control systems is that spurious pressure changes caused for example by the opening or closing of a door can sometimes be sufficient to result in undesired operation of a control system.

In order to overcome this problem in the installation shown 22, 23 and 24 of the sensing devices is very much in excess of the pressure required to operate the turbulence amplifiers H1, H2, H3, H4, and H5. When any particular nozzle is open there is thus a substantial stream of air flowing up through the appropriate aperture 27, 28 or 29. This substantial flow results in a small pressure change in the appropriate tapping 31, 32 or 33 and thus provides the desired order of pressure change to operate the appropriate turbulence amplifier.

Due to the fact that the tappings 31, 32 and 33 are well shielded from general air movements they tend to be unaffected by spurious signals and this enables accurate and effective control of the position of the web without interference from spurious air movements.

A further advantage of an apparatus made in accordance with the invention is that due to the substantial air flow there is very little tendency for particles of dirt to enter tappings 31, 32 and 33. Moreover the venting of the exhaust of the unit 43 to atmosphere through the valves 51, 52 also prevents contamination by dirt. Thus, incorrect operation due to the presence of dirt in the parts of the control means is unlikely to occur.

Iclaim:

1. In an apparatus for controlling the lateral position of a web of material, which apparatus comprises a sensing head through which the web is passed, first and second sensing elements in said sensing head arranged to sense the presence or absence of the web, means for supplying apressure medium, said sensing elements being so positioned as to define the limits of a range of permitted lateral deviation in the position of one longitudinal edge of the web, control means responsive to signals from said first and second sensing elements, and an adjusting device controlled by said control means to cause movement of the web to maintain the latter in its desired lateral position; i

the improvement comprising a third pneumatically operated sensing element positioned to sense the presence of the web at both of said limits of lateral devia tion and operably connected to said pressure medium supply means and to said control means, and said control means being constructed and arranged to inhibit operation of the adjusting device when receiving a signal from the third sensing element indicating absence of the web, said sensing head being in the form of a block having a slot therein through which thewebpasses, said pressure medium supply means, being a sourceof compressed air, and said third sensing element being in the form of an air emitting nozzle communicating with said slot, an aperture disposed in axial alignment with said nozzle and a tapping extending into said aperture to sense changes in pressure therein.

2. An apparatus according to claim 1, wherein the tappings are connected to turbulence amplifiers forming part of said control means which areeach such as to deliver an output pressure signal when the associated tapping senses a low pressure in its associated aperture.

3. An apparatus according to claim 2, wherein the control means comprises further turbulence amplifiers each controlled by the output of one of said first mentioned turbulence amplifiers.

4. An apparatus according to claim 3, wherein the outputs of two of the turbulence amplifiers control two step-up relays which in turn can move the piston of a double-acting piston and cylinder unit operably associated with said adjusting device.

5. An apparatus according to claim 4, wherein spring-return three-way valves are interposed between the piston and cylinder unit and the step-up relays to allow the exhaust of said unit to vent to atmosphere when the piston thereof is displaced by operation of one of said step-up relays.

6. An apparatus according to claim 4, wherein manually operable jogging valves are provided which are connected to said two turbulence amplifiers so as to directly control the operation thereof.

7. An apparatus according to claim 3, wherein the turbulence amplifiers are supplied with pressurized air from multiple outlets of a common manifold.

8. An apparatus according to claim 7, wherein the input of pressurized air to said manifold is by way of a filter and a pressure regulator.

9. An apparatus according to claim 5, wherein the turbulence amplifiers are supplied with pressurized air from multiple outlets of a common manifold, the output of which is fed with pressurized air from a filter and a pressure regulator, and the three-way valves are fed with pressurized air from a further regulator connected to said filter.

10. An apparatus according to claim 5, wherein the threeway valves are fed with pressurized air by way of a regulator and a filter.

11. An apparatus according to claim 9, wherein the exhaust outlet of each of the three-way valves communicates with the atmosphere via a pressure restrictor which controls the speed of displacement of the piston of said piston and cylinder unit.

12. An apparatus according to claim 1, wherein the pressurized air supplied to said nozzles is fed through a pressure restrictor.

13. An apparatus according to claim 1, wherein the control means has five turbulence amplifiers each with a pressurized air input and a control input, the presence or absence of which causes an output pressure signal to be absent or present respectively, the outputs of a first and a fifth of said amplifiers being used to control the operation of a double-acting piston and cylinder unit operably associated with said adjusting device, the tappings forming part of the first and the second missing sensing elements being connected to the control inputs of a fourth and a third amplifier respectively and the tapping forming part of the third sensing element being connected to the control input of the second amplifier and the control input of the fourth amplifier, and wherein the outputs of the second, third and fourth amplifiers are connected to the control inputs of the first, second and fifth amplifiers respectively.

14. An apparatus according to claim 13, wherein first and second manually operable jogging valves are provided having a pressurized air inlet and an outlet which is pressurized when the valve is manually actuated, the first valve having its output connected to the control inputs of the second and fifth amplifiers and the second valve having its output connected to the control inputs of the first and fourth amplifiers. 

1. In an apparatus for controlling the lateral position of a web of material, which apparatus comprises a sensing head through which the web is passed, first and second sensing elements in said sensing head arranged to sense the presence or absence of the web, means for supplying a pressure medium, said sensing elements being so positioned as to define the limits of a range of permitted lateral deviation in the position of one longitudinal edge of the web, control means responsive to signals from said first and second sensing elements, and an adjusting device controlled by said control means to cause movement of the web to maintain the latter in its desired lateral position; the improvement comprising a third pneumatically operated sensing element positioned to sense the presence of the web at both of said limits of lateral deviation and operably connected to said pressure medium supply means and to said control means, and said control means being constructed and arranged to inhibit operation of the adjusting device when receiving a signal from the third sensing element indicating absence of the web, said sensing head being in the form of a block haVing a slot therein through which the web passes, said pressure medium supply means being a source of compressed air, and said third sensing element being in the form of an air emitting nozzle communicating with said slot, an aperture disposed in axial alignment with said nozzle and a tapping extending into said aperture to sense changes in pressure therein.
 2. An apparatus according to claim 1, wherein the tappings are connected to turbulence amplifiers forming part of said control means which are each such as to deliver an output pressure signal when the associated tapping senses a low pressure in its associated aperture.
 3. An apparatus according to claim 2, wherein the control means comprises further turbulence amplifiers each controlled by the output of one of said first mentioned turbulence amplifiers.
 4. An apparatus according to claim 3, wherein the outputs of two of the turbulence amplifiers control two step-up relays which in turn can move the piston of a double-acting piston and cylinder unit operably associated with said adjusting device.
 5. An apparatus according to claim 4, wherein spring-return three-way valves are interposed between the piston and cylinder unit and the step-up relays to allow the exhaust of said unit to vent to atmosphere when the piston thereof is displaced by operation of one of said step-up relays.
 6. An apparatus according to claim 4, wherein manually operable jogging valves are provided which are connected to said two turbulence amplifiers so as to directly control the operation thereof.
 7. An apparatus according to claim 3, wherein the turbulence amplifiers are supplied with pressurized air from multiple outlets of a common manifold.
 8. An apparatus according to claim 7, wherein the input of pressurized air to said manifold is by way of a filter and a pressure regulator.
 9. An apparatus according to claim 5, wherein the turbulence amplifiers are supplied with pressurized air from multiple outlets of a common manifold, the output of which is fed with pressurized air from a filter and a pressure regulator, and the three-way valves are fed with pressurized air from a further regulator connected to said filter.
 10. An apparatus according to claim 5, wherein the three-way valves are fed with pressurized air by way of a regulator and a filter.
 11. An apparatus according to claim 9, wherein the exhaust outlet of each of the three-way valves communicates with the atmosphere via a pressure restrictor which controls the speed of displacement of the piston of said piston and cylinder unit.
 12. An apparatus according to claim 1, wherein the pressurized air supplied to said nozzles is fed through a pressure restrictor.
 13. An apparatus according to claim 1, wherein the control means has five turbulence amplifiers each with a pressurized air input and a control input, the presence or absence of which causes an output pressure signal to be absent or present respectively, the outputs of a first and a fifth of said amplifiers being used to control the operation of a double-acting piston and cylinder unit operably associated with said adjusting device, the tappings forming part of the first and the second missing sensing elements being connected to the control inputs of a fourth and a third amplifier respectively and the tapping forming part of the third sensing element being connected to the control input of the second amplifier and the control input of the fourth amplifier, and wherein the outputs of the second, third and fourth amplifiers are connected to the control inputs of the first, second and fifth amplifiers respectively.
 14. An apparatus according to claim 13, wherein first and second manually operable jogging valves are provided having a pressurized air inlet and an outlet which is pressurized when the valve is manually actuated, the first valve having its output connected to the control inputs of the second and fifth amplifiers and the second valve having its output connected to the control inputs of thE first and fourth amplifiers. 